Ethanolamines were first synthesized in a laboratory setting in 1860 when the pioneering Alsatian chemist Charles-Adolphe Wurtz heated ethylene chlorohydrin with aqueous ammonia in a closed tube. While never commercially interesting during the nineteenth century, ethanolamines were nonetheless enough of a technical curiosity that they attracted considerable technical interest. For example, the great German chemist Ludwig Knorr significantly improved upon Wurtz's work when in 1897 he successfully separated ethanolamines into their mono-, di- and triethanolamine component parts, as well as made other contributions to their synthesis.
Despite process improvements and continued laboratory interest, ethanolamines only attracted substantial commercial development after 1945. At this time, the significant increase in the industrial production of ethylene oxide was also leading to considerable interest in ethylene oxide derivatives. Ironically, this commercial movement from ethylene oxide to ethanolamines recapitulated the history of the synthesis of the chemicals as Wurtz's synthesis of ethanolamines in 1860 was largely the result of his trying to figure out what he could make with a new chemical he had discovered just the year before—ethylene oxide.
In the post-war years, significant process improvements were subsequently made as a result of the burgeoning interest in ethanolamines, which had proven to be extremely versatile intermediates in a wide variety of chemical products such as emulsifiers, surfactants, and agrichemicals, as well as many others. Examples of such improvement can be seen in, for example, U.S. Pat. No. 2,196,554 to Guinot which discloses an aqueous process with an improved heat integration and efficiency scheme for the concentration of ethanolamines in the process backend. Another example is GB Patent No. 760,215 to Lowe et al., which discloses that by controlling the molar ratios at which ammonia and ethylene oxide are mixed, then a higher content of di- or tri-ethanolamine may be obtained. Alternatively, GB Patent No. 1 529 193 to Gleich discloses that a higher di- or tri-ethanolamine content may be obtained by recycling di- or tri-ethanolamine to the reactor.
Given that the conversion of reactants to products is nearly complete in an ethanolamines process and the fact that the process has developed into a mature technology by process improvements such as those mentioned above, wringing out additional improvements or competitive technical advantages in ethanolamines technology has proved difficult. Opportunities for process improvement in ethanolamines reside mainly in product quality and thermal and utility efficiency. For example, reducing the amount of water used in the process would significantly improve the utility efficiency and process economics, since this means less water must be removed later in the process. Water content can only be reduced, without negatively affecting product quality, by maintaining lower temperatures. Product quality is related to temperature because higher temperatures in an ethanolamines process often lead to discoloration of the ethanolamines product. Thus, there has been a considerable effort to reduce the ratio of water to ammonia used in the process.
However, attempts to achieve an economically desirable ratio of water to ammonia have proved largely unsuccessful as doing this often leads to excess pressures in the bottom of the absorber (since as the ratio of water to ammonia in a solution decreases, the vapor pressure in the solution increases). If the absorber backpressures the rest of the recycle and recovery sections this can require that the temperatures in the bottom of the stripping column and evaporator columns be increased, which can in turn result in a loss of product quality. For example, U.S. Pat. No. 4,119,670 notes that the high temperatures required for ammonia stripping often results in a discolored alkanolamines product. U.S. Pat. No. 4,335,181 discloses a process for decreasing the water to ammonia ratio, but expresses concern that in doing so the temperature in the bottom of the stripping column should be no greater than 150° C.
Accordingly, there is a continuing need in the art for an ethanolamine manufacturing process with the improved process economics and efficiency of operating at high ammonia to water ratios and that also produces quality, on-spec product.